A telomerization reaction of a conjugated diene compound means an oligomerization reaction of a conjugated diene compound upon intake of a nucleophilic reactant. For example, a reaction wherein 2 molecules of butadiene react with 1 molecule of a compound having an active hydrogen such as acetic acid and the like to give a product such as 1-acetoxy-2,7-octadiene and the like can be mentioned.
It is known that a palladium complex, particularly, a palladium complex coordinating phosphines shows a superior activity as a catalyst for telomerization reaction of a conjugated diene compound [see, for example, Palladium Reagents and Catalysts, Written by Jiro Tsuji, Published by John Wiley & Sons, pp. 423-441 (1995)]. However, use of a palladium complex coordinating phosphines as a catalyst for industrial telomerization reaction is associated with the following problems. (1) A palladium complex coordinating phosphines shows poor thermal stability, and when a telomerization product and a catalyst component are separated by evaporation, the complex is decomposed during the step to precipitate palladium metal. As a result, the catalyst cannot be reused easily and the precipitated metal causes problems of piping blockage and the like. (2) To maintain thermal stability of the palladium complex coordinating phosphines, the reaction mixture should contain an excess amount of phosphine per 1 atom of the palladium. While the presence of an excess amount of phosphine enhances the stability of the palladium complex coordinating phosphines, it degrades the catalytic activity. Moreover, problems such as decrease of the concentration of phosphine due to the production of phosphine oxide as a result of the oxidization of the excess phosphine, degraded catalytic activity and the like occur. From the above aspects, a compound having a coordinating property to a metal such as palladium and the like and exhibiting a telomerization reactivity has been demanded as a ligand replacing phosphine.
A method has been reported wherein a conjugated diene compound having 4 to 6 carbon atoms and a mono-alcohol are telomerized in the presence of a catalyst system comprising isocyanide, which is a non-phosphine type ligand, and a nickel compound to give unsaturated ether [see, for example, U.S. Pat. No. 3,670,029]. In Examples thereof, a telomerization reaction of 1,3-butadiene and methanol is carried out in the presence of a catalyst system comprising bis (1,5-cyclooctadiene) nickel and cyclohexylisocyanide [0.001-0.01 equivalent of bis (1,5-cyclooctadiene) nickel relative to 1,3-butadiene], whereby 1-methoxy-2,7-octadiene and 3-methoxy-1,7-octadiene are obtained at a ratio of 91:9 (weight ratio).
When a catalyst system comprising a nickel compound and isocyanide is used for telomerization reaction of a conjugated diene compound and alcohol, the terminal-position selectivity of the alcohol-addition product is about 90% at the highest. Since the yield of substantial 1-position substituted ether is low and the catalytic activity is low, a large amount of catalyst is necessary, thus insufficient in the industrial method.
In addition, a telomerization reaction using a palladium catalyst comprising nitrogen-containing heterocyclic carbene as a non-phosphine ligand has been reported [see, for example, DE 10128144 A1 and Angew. Chem. Int. Ed., vol. 41, pp. 1290-1309 (2002)]. A nitrogen-containing heterocyclic carbene has high electron-donating property and firmly binds with a metal. A metal coordinating with the nitrogen-containing heterocyclic carbene shows a remarkably increased electron density. Therefore, a palladium complex coordinating nitrogen-containing heterocyclic carbene is superior in thermal stability and shows superior catalytic activity in oxidative addition reaction and the like. Such palladium complex is known to be usable as a catalyst of coupling reactions such as Mizorogi-Heck reaction using aryl chloride, Suzuki-Miyaura coupling reaction and the like [see, for example, Platinum Metals Rev., vol. 46, pp. 50-64 (2002) and Advances in Organometallic Chemistry, vol. 48, pp. 42-47 (2002)]. It has been reported that, when this complex is used as a catalyst of telomerization of 1,3-butadiene and methanol, the complex shows superior productivity (TON, turnover number), selectivity of terminal-position of the methanol addition product and telomerization selectivity, as compared to a palladium complex coordinating phosphine [see, for example, Angew. Chem. Int. Ed., vol. 41, pp. 986-989 (2002) and Journal of Molecular Catalysis A: Chemical, vol. 185, pp. 105-112 (2002)].
In telomerization using a palladium complex coordinating nitrogen-containing heterocyclic carbene, as mentioned above, the rate of an oxidative coupling reaction of 2 molecules of a conjugated diene compound increases due to the electron-donating property of the nitrogen-containing heterocyclic carbene, but the rate of a reductive elimination reaction becomes slow. To enhance reaction efficiency, an excess base needs to be added to the palladium complex coordinating nitrogen-containing heterocyclic carbene. Consequently, a problem occurs in that the stability of a palladium complex coordinating nitrogen-containing heterocyclic carbene cannot be maintained easily. Moreover, it is assumed that, when such telomerization is industrially performed and the catalyst is circulated for reuse, the catalytic activity will be degraded, and serious problems of corrosion of reaction reactor, piping blockage due to precipitation of a base and the like will be produced. An additional problem is expected in that the cost of ligand becomes high because several steps are required to separately synthesize nitrogen-containing heterocyclic carbene to be used as a ligand.
Furthermore, a telomerization reaction using a palladium catalyst, wherein isocyanide is used as a non-phosphine ligand, has been reported (see, for example, JP S48-43327 B, U.S. Pat. No. 3,670,032). In an Example therein, a telomerization reaction of 1,3-butadiene and trimethylolpropane is carried out in the presence of a catalyst system of tetrakis (triphenylphosphine)palladium and cyclohexylisocyanide. However, the ratio of tetrakis (triphenylphosphine)palladium as the catalyst and butadiene used, yield of the product, reaction time and the like are not described but it has been only reported that octadienyldihydroxymethylbutane was obtained as a main product. The present inventors have found that when an isocyanide having hydrogen on the carbon at the α-position thereof is used as a ligand, the hydrogen is eliminated by a base used for the telomerization reaction, thus resulting in the decomposition of isocyanide and a failure to show an intended function of a ligand, and the object function as a telomerizing catalyst ligand is markedly degraded (below-mentioned Comparative Example 2). In the reported Example, moreover, isocyanide is concurrently used with a palladium catalyst already having phosphorus ligands. The present inventors have found problems associated with the use of a palladium already having phosphorus ligands, in that it suppresses coordination of isocyanide, thereby markedly lowering the rate of the reaction, and degrades the regioselectivity of alcohol in nucleophilic reaction (below-mentioned Comparative Example 3), and therefore, the terminal-position selectivity, i.e. straight chain selectivity of alcohol addition becomes low. The present inventors have also found a problem in that the amount of a catalyst to be used per butadiene cannot be reduced, or the conversion ratio of butadiene cannot be increased, because coordination of phosphorus prevents sufficient increase in the charge density on palladium.